This invention relates generally to calcined clay products, and more specifically relates to a method for treating a substantially anhydrous white kaolin clay powder so as to improve the bulk handling characteristics of same when the product is loaded, unloaded and shipped.
In the course of manufacturing paper and similar products, including paper board and the like, it is well-known to incorporate quantities of inorganic materials into the fibrous web in order to improve the quality of the resulting product. A number of inorganic materials have long been known to be effective for these purposes, such as titanium dioxide, which can be incorporated into the paper in the form of anatase or rutile. Titanium dioxide, however, is among the most expensive materials which are so usable. Accordingly, in recent years, considerable efforts have been made to develop satisfactory replacements for the said titanium dioxide.
Among the materials which have thus found increasing acceptance as paper fillers are substantially anhydrous kaolin clays. Materials of this type are generally prepared by partially or fully calcining a crude kaolin clay, which may have been initially subjected to prior beneficiation steps in order to remove certain impurities, e.g. for the purpose of improving brightness in the ultimate product. As used in this specification, the term "substantially anhydrous kaolin clay powder" shall include kaolin clays which have been heated to over 400.degree. C. to render same anhydrous. The term thereby embraces (1) fully calcined kaolins--which usually have been heated above the 980.degree. C. exotherm, as well as so-called (2) "metakaolin", which results from heating to lower temperatures--below the exotherm. Reference may be had in the foregoing connection to Proctor, U.S. Pat. Nos. 3,014,836 and to Fanselow et al, 3,586,823, which disclosures are representative of portions of the prior art pertinent to fully calcined kaolins; and to Morris, U.S. Pat. Nos. 3,519,453, to Podschus, 3,021,195 and 3,309,214, and to British Pat. No. 1,181,491, all of which are concerned with kaolins which are calcined to lower temperatures and which therefore can be regarded as metakaolins.
A calcined product having characteristics generally superior to previously available such pigments, is the ALPHATEX.RTM. product of Anglo-American Clays Corporation, assignee of the present application. This product again is a substantially anhydrous white kaolin clay pigment, which has unusual efficacy as a filler in paper sheets and similar paper products. The pigment also has application as a coating pigment for paper, and as a pigment in paints and other filled systems. It generally consists of aggregates of anhydrous kaolin clay particles, and exhibits exceptionally high light-scattering and opacifying characteristics when incorporated as a filler in paper. The said pigment is a powdered material of quite fine size--typically at least 65% by weight are of less than 2 microns equivalent spherical diameter (ESD). The said pigment exhibits a Valley abrasion value of less than 50 mg. and usually below 30 mg., (as determined by the Institute of Paper Chemistry Procedure 65).
ALPHATEX.RTM. is further described in U.S. Pat. No. 4,381,948 to A. D. McConnell et al, as being an anhydrous white kaolin clay pigment having high light scattering when incorporated as a filler in paper, the pigment consisting of porous aggregates formed from sub-micron sized kaolin clay platelets obtained by classification of a dispersed kaolin clay to a 100% less than one micron ESD fraction, the aggregates having an average specific gravity in the range of 0.5 to 0.6 and a mean internal pore size of less than 0.55 microns. The size distribution of the aggregates is such that not more than 5% by weight thereof are greater than 10 microns ESD, at least 75% are of less than 2 microns ESD, and not more than 15% by weight are of less than 1 micron ESD. The pigment has a Valley abrasion value below 30 mg, and a GE brightness of at least 93.
Calcined kaolin clay products such as ALPHATEX.RTM. are normally pulverized in a high energy impact mill and air-classified after calcination for the purpose of removing +325 mesh residue (to conform to specification for intended use in paper), or in order to remove larger abrasive particles. Such products are then sold by the manufacturer as a finally pulverized low-bulk density powder, which powder is extremely difficult to handle by conventional bulk handling systems. Because of the difficulties in handling such products, they are typically shipped in bulk in "sparger cars". These are bulk-hopper railroad cars fitted with special valves at the bottom which allow water to be injected into the car upon arrival at the customer's facility. Air is then injected into the car to agitate the water and powdered clay mixture. As soon as possible after the injection of the water and air, the fluid suspension is pumped from the car at about 30%-35% solids into a storage tank.
Because of the low-bulk density of the clay powders, typically only 35 to 40 tons of clay can be loaded into a 4,700/ft.sup.3 rail car. The bulk density of this material would be measured in the laboratory to be about 10 to 12 lbs/ft.sup.3 ; this material would pack to about 15 to 17 lbs/ft.sup.3 in a fully loaded railroad car.
A further serious problem encountered when using the bulk sparger cars is the difficulty of mixing the dry-powdered calcined kaolin clay powder with water in a car having only air agitation available as a means of mixing.
A laboratory test has been developed which measures the ease with which the dry powder can be admixed with water. This test consists of placing a known volume of water in a beaker, then pouring a known weight of calcined clay on top of the water in the beaker with no agitation of any type, and measuring the time it takes for the clay to drop below the surface of the water. The specific test hereinafter referred to in this specification as the "wet-out test" is performed as follows: 100 grams of tap water are poured into a 600 ml. beaker. 50 grams of calcined clay are then poured into the beaker while simultaneously starting a stopwatch. As soon as all of the clay has disappeared under the surface of the water without any agitation, time is noted. The result is reported in terms of time, i.e. seconds.
The wet-out test just described, can be correlated or supplemented with a further test procedure which measures a quantity designed the "work index" ("WI"). In this test, a predetermined quantity of the dry powder to be tested is deposited in a predetermined amount of water, after which the slurry is mixed at a specific set speed with a standard mechanical mixer. A parameter indicative of the resistance encountered by the mixer blades, and thus of the viscosity of the slurry is observed and plotted as a function of time. The plot is a generally bell-shaped curve which, however, flattens into a relatively level "tail" when a fully stable slurry is achieved. The point of flattening out corresponds to completion of wet-out of the powder; and the area under the curve up to that point is a measure of energy input to achieve wet-out, and is designated the "work index" for the material tested.
A "tapped" bulk density measurement procedure is used in the laboratory, and is performed as follows: A pretared 100 ml cylinder is completely filled and tapped lightly until the level in the cylinder ceases to drop fairly rapidly. The level is then adjusted to 100 ml and container and clay are weighed. The bulk density quoted in lbs/ft.sup.3 is then calculated as follows: ##EQU1##
In the past, efforts have been made to increase the bulk density of the calcined powders using compaction equipment, such as bricketting machines or pelletizers. However, these have proved to be unacceptable for several reasons. Among these is that bricketting machines tend to produce hard agglomerates, which are difficult to redisperse in water. This causes problems at the paper manufacturer's slurry make-down facility.
Further, pelletizing equipment which relies upon water as a binder has been found to require the addition of large quantities of water (roughly 40% of the weight of the clay) before acceptable pellets can be formed. This water either increases the shipping costs of the product or increases production costs in that it must be evaporated prior to shipment. Pelletizing equipment relying upon binders other than water also requires large amounts of binder and are found to result in a pelletized product which is difficult to make-down in water after pelletization and drying.
It has been found that one means of improving the wet-out rate of calcined kaolin clay powders is by the addition of dispersant or surfactant in dry form to the dry powder. For example, the addition of 5 pounds dry sodium hexametaphosphate to a ton of calcined kaolin clay powder will reduce the wet-out time from about 160 seconds to about 80 seconds. This method of improving the wet-out rate is expensive, however, and does nothing to increase the bulk density of the powder.
The wet-out rate can also be improved on a laboratory scale by grinding the powder in a small lab grinder using approximately 300 HP hrs energy input per ton of clay. In this way, it has been found that the wet-out rate can be reduced substantially.